The Collaborative International Dictionary of English v.0.48:

Equator \E*qua"tor\, n. [L. aequator one who equalizes: cf. F.
   ['e]quateur equator. See Equate.]
   1. (Geog.) The imaginary great circle on the earth's surface,
      everywhere equally distant from the two poles, and
      dividing the earth's surface into two hemispheres.
      [1913 Webster]

   2. (Astron.) The great circle of the celestial sphere,
      coincident with the plane of the earth's equator; -- so
      called because when the sun is in it, the days and nights
      are of equal length; hence called also the equinoctial,
      and on maps, globes, etc., the equinoctial line.
      [1913 Webster]

   Equator of the sun or Equator of a planet (Astron.), the
      great circle whose plane passes through through the center
      of the body, and is perpendicular to its axis of
      revolution.

   Magnetic equator. See Aclinic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

equator
    n 1: an imaginary line around the Earth forming the great circle
         that is equidistant from the north and south poles; "the
         equator is the boundary between the northern and southern
         hemispheres"
    2: a circle dividing a sphere or other surface into two usually
       equal and symmetrical parts

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "equator":
   Antarctic Zone, Arctic Circle, Arctic Zone, Frigid Zones,
   Torrid Zone, Tropic of Cancer, Tropic of Capricorn, Variable Zones,
   anklet, aphelion, apogee, armlet, astronomical longitude,
   autumnal equinox, band, belt, bisector, boundary, bracelet,
   celestial equator, celestial longitude, celestial meridian, center,
   cincture, cingulum, circle, climate, clime, collar, collarband,
   colures, core, diameter, diaphragm, divider, earring, ecliptic,
   equinoctial, equinoctial circle, equinoctial colure, equinox,
   fascia, fillet, finger ring, furnace, galactic longitude,
   geocentric longitude, geodetic longitude, girdle, girt, girth,
   great circle, halfway mark, heart, heliocentric longitude, hell,
   hoop, horse latitudes, inferno, interior, kernel, latitude,
   line of demarcation, longitude, longitude in arc, mean, median,
   meridian, middle, midmost, midriff, midst, neckband, necklace,
   nose ring, nucleus, orbit, oven, parallel, partition, perigee,
   perihelion, period, prime meridian, quoit, ring, roaring forties,
   small circle, solstitial colure, steam bath, subtropics, the line,
   thick, thick of things, trajectory, tropic, tropics,
   vernal equinox, waist, waistline, wristband, wristlet, zodiac,
   zone